MCQs in Medical Physiology - E.S.Prakash

[Pages:138]Multiple-Choice Questions in Medical Physiology

For Postgraduate Medical Entrance Examinations Revised 2014

E.S.Prakash, MBBS, MD

Written and published by E.S.Prakash. Copyright ? 2014, all rights reserved by E.S.Prakash. Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means or stored in a database or retrieval system without prior written permission from the author.

Disclaimer: Science and medicine constantly change and while I have tried to check information presented here with sources known to be reliable, I cannot guarantee that this book is error free. I encourage you to crosscheck information contained herein with other sources. This book is not written to be used to directly guide the care of patients. Therefore, I cannot accept any responsibility for any consequences that may arise from the use of information contained here in the care of patients. ? E.S.Prakash

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Table of Contents

Chapter Frequently used abbreviations 2 minutes please General Physiology Nerve, Muscle, Synaptic Physiology and Neurotransmission Central Nervous System Endocrinology and Reproduction Gastrointestinal Physiology Blood and Cardiovascular Physiology Pulmonary Physiology Renal and Acid-Base Physiology

Pages 2 3 4 22 37 54 72 81 107 123

Frequently used abbreviations aka ? also known as BP ? blood pressure CSF ? cerebrospinal fluid ECF ? extracellular fluid Hb ? hemoglobin Hct ? hematocrit ICF ? intracellular fluid ISF ? interstitial fluid MAP ? mean systemic arterial pressure MCQ ? multiple-choice question PCV ? packed cell volume RBC ? red blood cell SVR ? systemic vascular resistance T/F ? True or false TPR ? total peripheral resistance (aka. Systemic vascular resistance) WBC ? white blood cell WNL ? within normal limits

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2 minutes please

Think of this book as a tool to help you assess how well you have learnt medical physiology. It has been written primarily for use in revision courses for doctors who are preparing for competitive postgraduate medical entrance examinations in India, but undergraduate medical students and postgraduate physiology students, may also find this helpful. The intent here is to use multiple-choice questions (MCQ) as a means to help the reader revise key facts, test understanding of concepts and the ability to apply them.

Many questions that I have seen appear on entrance examinations test conceptual understanding and meaningful learning. However, depending on the exam, some test knowledge of facts that are not of significance to a practicing primary care physician, and an undesirable backwash effect of this is it tends to encourage rote learning over indepth learning of important concepts. In writing this book, I have focused on the immediate `learning needs' of the target audience, which is to be successful on these exams.

I've frequently included questions solely for learning and systematic revision of certain topics. Thus, not all questions are written to be equivalent in standard to questions commonly appearing in entrance examinations. That being said, the difficulty level of questions varies from one exam to another with some predominantly testing recall of facts whereas others test knowledge and understanding and the ability to apply concepts and solve problems. Just to be clear, no attempt is made here to be comprehensive. Summaries of certain topics can be found at the start of each section. I'd suggest giving the summaries a quick read before doing questions and explanations (for questions) that may be found in summaries upfront are not repeated.

As MCQs with multiple correct answers enable more material to be revised with fewer questions, I have also frequently used such questions; this helps us get into and sustain the important habit of carefully reading all options in a question. Furthermore, some entrance exams use the multiple true-false type of MCQ, so some practice with this type of question is needed.

For the purpose of preparing and revising for PG entrance examinations, I recommend: Ganong's Review of Medical Physiology by Barrett et al, published by Mc Graw Hill, 2012.

I continue to revise and publish Multiple-Choice Questions in Medical Physiology on a regular basis, and I welcome suggestions from readers for improving it. If you spot an error, please let me know. If any corrections are needed, I'll post them on my website at as soon as they are identified.

This book is dedicated to the memory of Dr. William F. Ganong.

E.S.Prakash, MBBS, MD Associate Professor of Physiology Mercer University School of Medicine, Macon, Georgia, USA E-mail: elapulli.prakash@ Web:

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GENERAL PHYSIOLOGY

Body composition: Body mass = fat mass + lean body mass.

The water content of lean body mass (fat-free mass) is relatively constant and is about 70 ml/100 g. For example, in a healthy adult male weighing 70 kg and whose total body water is measured to be 42 L (= 42 kg), lean body mass = 42 / 0.7 = 60 kg, and fat mass is 10 kg. Body fat % = 10/70 ? 100 = 14%. Fat is relatively anhydrous. Body fat percentage is greater in women compared to men.

Body Fluid Compartments:

Typical values in a healthy adult male weighing

70 kg are as given below:

Compartment Volume

Marker

Total body

42 L

D2O

water

ICF

28 L

-

ECF

14 L

Sucrose

Interstitial

10.5 L

-

fluid

Plasma

3.5 L

Evans blue

Radioiodinated

albumin

Intravenously administered sucrose distributes throughout ECF (`sucrose space') and it does not enter cells. ECF volume in a 70-kg healthy adult is about 14 L (20% of body weight). The volume of interstitial fluid is about 10.5 L (75% of ECF volume) and plasma volume is about 3.5L (25% of ECF volume). Heavy water distributes throughout body water. Inulin and mannitol distribute exclusively in the ECF. Evans blue dye stays in the plasma, and radioiodinated albumin has also been used to estimate plasma volume.

Calculating Serum Osmolality: Serum osmolality is calculated in mosm/L as equal to:

2 [Na+] + [glucose] / 18 + [BUN] / 2.8,

where serum Na is expressed in mmol/L and serum glucose and serum urea nitrogen are expressed in mg/dL.

Blood urea vs. blood (serum) urea nitrogen: The formula of urea is NH2CONH2 Molar mass of urea is 60 g; each molecule of urea has 2 nitrogen atoms. The mass of nitrogen in urea is 2 ? 14 = 28 g Thus, a urea concentration of 60 mg/dL corresponds to a [BUN] of 28 mg/dL.

Normally, the osmolality of serum or plasma is chiefly due to Na and its accompanying anions Cl and HCO3. We multiply Na by 2 to account for obligate anions accompanying Na. Normally, serum osmolality ranges from 280?295 mOsm/Kg H2O. The osmolality of body fluids can also be directly measured using the freezing point depression method.

The concentration of sodium in ECF is quantitatively the most important determinant of plasma osmolality.

Relationship between blood volume and plasma volume:

Blood volume = plasma volume ? [100 / (100?Hct)]

The relationship between blood flow and plasma flow through an organ is similar.

For example, renal blood flow = renal plasma flow ? [100 / (100?Hct)]

ICF is much more acidic than ECF. For example, in muscle cells the pH is typically about 6.8. Steady state osmolality (i.e., concentration of osmotically active particles) of all body fluid compartments must be the same. The fact that, in a healthy adult, ICF volume is twice as large as ECF volume indicates that the absolute number of osmoles is much greater in the ICF.

What is osmosis? Osmosis is the movement of water across a semipermeable membrane permeable to water but not to solutes, from a solution with lower concentration of osmoles to a solution with a higher concentration of osmoles. This continues until osmotic equilibrium (i.e., the osmolality of either compartment is equal) is attained.

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E.S.Prakash. Multiple-Choice Questions in Medical Physiology, 2014

What is an osmole? An osmole (effective osmole) is an osmotically active particle; i.e., capable of causing osmosis. Examples include sodium ion, chloride ion, protein anions, and phosphate ion. A solute to which the cell membrane is relatively impermeable will function as an effective osmole. For example, in comparison to water, the cell membrane is relatively impermeable to sodium, chloride, mannitol. The amount of ions crossing the cell membrane through channels and transporters is much smaller relative to the magnitude of osmotically driven water fluxes.

What is an ineffective osmole? If the cell membrane is significantly permeable to a solute (for example, urea), the substance will move across the membrane until its concentration is exactly the same on both sides of the membrane. In such an instance, osmosis (net movement of water) does not occur. However, urea does function as an effective osmole in the renal medullary interstitium because the permeability of the inner medullary collecting ducts to urea is mediated by urea transporters dependent upon stimulation by antidiuretic hormone.

Osmolality is a colligative property that depends upon the number of solute particles, and not the size of the particles. As an example, the contribution of 1 Na ion and 1 albumin molecule toward the osmolality of plasma is the same. Since the molar concentration of proteins in plasma is very low (60 g/L) compared to that of Na (140 mM) and Cl (100 mM), plasma proteins contribute very little to the osmolality of plasma compared to Na and its accompanying ions.

Colloid osmotic pressure of plasma (oncotic pressure): While proteins are present in plasma at a concentration of 60-80 g/L, they are not normally present in significant concentrations in the interstitium. Thus, the osmotic pressure of plasma proteins (called colloid osmotic pressure or oncotic pressure) is much greater than the osmotic pressure of proteins in the interstitium. This oncotic pressure gradient across the capillary restrains fluid filtration and favors reabsorption of fluid into the capillary.

Hemolysis begins when normocytes are placed in 0.5% NaCl and is complete in 0.3% NaCl. In contrast, when RBCs are placed in hypertonic saline, they lose water and diminish in size.

Effective osmolality of plasma: Since urea equilibrates across the cell membrane in the steady state, an increase in serum urea nitrogen does not cause cellular dehydration. The effective osmolality of plasma (in mOsm/L) is estimated as 2 [Na+] + [glucose] / 18 where Na is in mmol/L and glucose is expressed in mg/dL.

Osmotic pressure is the pressure required to stop water flux (osmosis) across a semipermeable membrane.

Why is 0.9% NaCl called an isotonic solution? 0.9% (precisely 0.85%) NaCl has the same osmolality as normal human plasma (about 290 mOsm/kg H2O, see the calculation below). When it is infused into an individual with an ECF osmolality of 290 mOsm/kg H2O, it does not cause any change in the steady state volume of red blood cells or other cells ? it is an isotonic solution. Tonicity of a solution refers to the effect of a solution on the osmolality of normal human plasma in the steady state,

Osmotic pressure P = CRT (Van't Hoff equation), where, C is concentration of osmoles; R is a constant, and T is temperature in Kelvin

In the steady state, the osmolality of all body fluids is identical; that is, osmotic pressure gradient across the plasma membranes of cells in the steady state is zero.

Calculate the osmolality of 0.85% NaCl. 0.85% NaCl contains 0.85 g of NaCl per deciliter of the solution = 8.5 g/ L of the solution Molar mass of NaCl = 58.5 g; 1 mol of NaCl contains 58.5 g of NaCl 8.5 g corresponds 8.5/58.5 mol = 0.145 mol = 145 mmol/L Each Na in this solution is accompanied by 1 chloride ion. Therefore, the total concentration of

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E.S.Prakash. Multiple-Choice Questions in Medical Physiology, 2014

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